The invention relates to a method for generating video holograms, in particular computer-generated video holograms (CGVH), from image data with depth information in real time. During the holographic reconstruction of the three-dimensional objects or three-dimensional scenes, the light wave front is generated through interference and superimposition of coherent light waves.
In contrast to classic holograms, which are stored photographically or in another suitable way in the form of interference patterns, video holograms exist as a result of the computation of hologram data from sequences of a three-dimensional scene and of their storage with electronic means.
In a holographic display device, modulated light which is capable of generating interference propagates in the space in front of the eyes of an observer in the form of a light wave front which is controllable through amplitude and/or phase values, said light wave front thereby reconstructing a three-dimensional scene. Controlling a light modulator means with the hologram values of the video holograms causes the emitted wave field, which has been modulated in its pixels, to reconstruct the desired three-dimensional scene in the space by creating interferences.
A holographic display device typically comprises an arrangement of controllable pixels which reconstruct object points by electronically influencing the amplitude and/or phase of illuminating light. In this document, the term ‘pixel’ denotes a controllable hologram pixel in the light modulator means; a pixel is individually addressed and controlled by a discrete value of a hologram point. Each pixel represents a hologram point of the video hologram. In an LCD, the term ‘pixel’ is therefore used for the individually addressable image points of the display screen. In a Digital Light Processing display (DLP), the term ‘pixel’ is used for an individual micro-mirror or a small group of micro-mirrors. In a continuous SLM, a ‘pixel’ is the transitional region on the light modulator means which represents a complex hologram point. The term ‘pixel’ thus generally denotes the smallest unit which represents or which is able to display a complex hologram point.
Many types of light modulator means are known, for example in the form of a spatial light modulator (SLM). The light modulator means can be of a continuous type or of a matrix type. For example, it may be a continuous SLM with a matrix control or an acousto-optic modulator (AOM). A liquid crystal display (LCD) serves as an example of such a suitable display device for the reconstruction of video holograms by way of amplitude modulation of a light pattern. However, this invention can also be applied to other controllable devices which use coherent light for modulating a light wave front.
A holographic display device which is preferably used for the present invention is substantially based on the following principle: A scene which is divided into object points is encoded as a total hologram on at least one light modulator means. The scene can be seen as a reconstruction from a visibility region which lies within one periodicity interval of the reconstruction of the video hologram. A sub-hologram is defined for each object point of the scene to be reconstructed. The total hologram is formed by a superimposition of sub-holograms. In general, the principle is to reconstruct mainly that wave front that would be emitted by an object into one or multiple visibility regions. The reconstruction of a single object point only requires a sub-hologram as a subset of the total hologram which is encoded on the light modulator means. The holographic display device comprises at least one screen means. The screen means is either the light modulator itself where the hologram of a scene is encoded, or an optical element—such as a lens or a mirror—on to which a hologram or wave front of a scene encoded on the light modulator is projected.
The definition of the screen means and the corresponding principles for the reconstruction of the scene in the visibility region are described in other documents filed by the applicant. In documents WO 2004/044659 and WO 2006/027228, the screen means is the light modulator itself. In document WO 2006/119760, “Projection device and method for holographic reconstruction of scenes”, the screen means is an optical element on to which a hologram which is encoded on the light modulator is projected. In document DE 10 2006 004 300, “Projection device for the holographic reconstruction of scenes”, the screen means is an optical element on to which a wave front of the scene encoded on the light modulator is projected.
The visibility region is a confined region through which the observer can watch the entire reconstructed scene. Within the visibility region, the wave fields interfere to form a wave front such that the reconstructed scene becomes visible for the observer. The visibility region is located on or near the eyes of the observer. The visibility region can be moved in the directions X, Y and Z and is tracked to the actual observer position with the help of known position detection and tracking systems. It is possible to use two visibility regions for each observer, one for each eye. Generally, other embodiments of visibility regions are also possible. It is further possible to encode video holograms such that for the observer individual objects or the entire scene seemingly lie behind the light modulator.
A virtual, frustum-shaped reconstruction space stretches between the light modulator means of the holographic display device and the visibility region, where the light modulator represents the base and the visibility region the top of the frustum. If the visibility regions are very small, the frustum can be approximated as a pyramid. The observer looks through the visibility region towards the holographic display device and receives in the visibility region the wave front which represents the scene.
Document WO/2006/066906 filed by the applicant describes a method for computing video holograms. It generally includes the steps of slicing the scene into section planes which are parallel to the plane of a light modulator, transforming all those section planes into a visibility region and adding them up there. Then, the added results are back-transformed into the hologram plane, where also the light modulator is disposed, thus determining the complex hologram values of the video hologram.
This method substantially carries out the following steps, aided by a computer, for a three-dimensional scene:
a diffraction image is computed in the form of a separate two-dimensional distribution of wave fields for an observer plane, which is situated at a finite distance and parallel to the section planes, from each object data set of each tomographic scene section, where the wave fields of all sections are computed for at least one common visibility region,
the computed distributions of all section planes are added so as to define an aggregated wave field for the visibility region in a data set which is referenced in relation to the observer plane, and
the reference data set for generating a hologram data set for a common computer-generated hologram of the scene, is transformed into a hologram plane, which is situated at a finite distance and parallel to the reference plane, where the light modulator means lies in the hologram plane.
The generation of the complex hologram values according to document WO/2006/066906 is very complex. Due to the large number of necessary transformations, the implementation of this method causes great computational loads.
Real-time encoding or generation of the hologram values would require costly high-performance computing units. Such expensive computing units would limit or impair the acceptance of digital video holography.